Tungsten has become an increasingly popular material for semiconductor integrated circuit metallization. Tungsten has often been found superior to aluminum for the filling of vias and/or windows. There are two commonly used methods for forming tungsten. The first method, often termed "blanket tungsten" involves the reaction of tungsten hexaflouride (WF.sub.6) and silane on an underlying nucleating substrate layer of TiN or TiW. The reaction produces a blanket layer of tungsten upon a substrate surface. The second method for forming tungsten, often termed "selective tungsten" involves the reaction of tungsten hexaflouride with silicon--typically a crystalline silicon substrate upon which integrated circuits are formed.
Those who employ selective tungsten to form a tungsten contact to a source/drain region (or other active silicon semiconductor region), often discover the subsequent formation of "worm holes." A worm hole is a microscopic tunnel formed in the silicon. The worm hole usually contains a small amount of tungsten material at its remote end. The presence of worm holes can destroy semiconductor junction integrity.
In an attempt to alleviate the worm hole problem, designers frequently deposit one or more barrier or glue layers within a window. The barrier or glue layer covers the exposed silicon of the source/drain region and helps to avoid the formation of worm holes. However, the presence of the barrier or glue layer precludes the use of selective tungsten because the crystalline silicon substrate is covered by the barrier or glue layer and therefore unavailable for reaction with WF.sub.6. Consequently, a blanket tungsten reaction must be employed if a barrier/glue layer is used in a window.
A typically used barrier/glue layer is a bilayer of titanium/titanium nitride. Titanium is deposited within the window first. Then a layer of titanium nitride is formed atop the titanium. Next, a blanket tungsten layer is formed in contact with the titanium nitride layer. However, the first-deposited titanium layer forms a silicide with the underlying silicon substrate. The silicide formation may be undesirable if the underlying junction is extremely shallow.
Designers have consistently sought better methods of tungsten deposition.